The purpose of this study is to evaluate the safety and effectiveness of two doses of the investigational drug h5G1.1-mAb as compared to placebo (inactive substance, dummy medication) in reducting proteinuria (protein in the urine) in patients with idiopathic membranous glomerulopathy. There has currently been limited experience with this investigational drug in membranous nephropathy, but it is currenly being studied in other medical conditions.